Polyurethane products, such as polyurethane foams, are produced by the reaction of a polyol and a polyisocyanate, in the presence of a catalyst and additives such as a flame retardant, a surfactant, pigments, dye, fillers, or other components. For foams, the use of a blowing agent is required. By varying the type and amount of reactants and optional additives, a wide variety of polymers and foams can be produced having different densities, rigidity, and structures.
The metering and mixing of two components (polyol and polyisocyanate) to produce a polyurethane product (referred to for foams as two components foams or TCF) being unpractical in many applications, one component polyurethane precursors (referred to for foams as one component foams or OCF) were developed, wherein a prepolymer containing isocyanate groups is filled into a pressure vessel. OCF's are sold in pressurized cans containing said prepolymer containing isocyanate groups together with one or more blowing agents and conventional additives. A polyurethane foam can then be produced by simply spraying the content of the pressure vessel, which will form a cellular structure under the action of the blowing agent and cure under the action of the ambient moisture. OCF's are of course much easier to use then TCF's since they require no metering, no mixing of the foam components and are currently used in e.g., the building sector, for example, for filling, insulating or fastening doors, windows and the like, and in the automotive industry, for vibration and sound dampening.
The literature disclosing one component foam compositions is abundant and the following patents can be cited as example only: EP480342; EP1674492; DE102008034272; FR2396035; GB962951; U.S. Pat. Nos. 3,830,760; 4,258,140; 4,508,853; 4,742,087; 5,817,860; 5,075,407; 5,786,402; 6,410,609; ; US200727051; WO02090410; WO02079291; U.S. Pat. Nos. 6,090,307; 5,631,319; US2007/0197672; WO00/04069; WO02/090410; WO2005/007721; WO2007/100502.
A typical polyisocyanate component present in most OCF compositions is diphenylmethane diisocyanate (MDI) or a mixture of diphenylmethane diisocyanate and poly-(phenylene methane poly-isocyanate), often referred to as crude MDI, which provides a convenient source of free NCO-groups to react with the polyols. The polyols used are usually a mixture of bi-, tri-, and sometimes higher functional polyols of rather high molecular weight, which react with MDI to form NCO-containing prepolymers. Each prepolymeric chain contains a limited number of free NCO-groups, of the order of 2 or 3 per chain, yielding a free NCO-group content of about 6-7% for rigid foams, and lower for more flexible chains. It is well established, however, that the optimal content of free NCO-groups to obtain a satisfactory crosslinking density is around 14.5% or more. In order to reach the required proportion of free NCO-groups, most compositions comprise an excess amount of free MDI. The excess content of MDI in the mixture is further encouraged by the fact that MDI is a good solvent for NCO-containing prepolymers, which need be dissolved lest the viscosity of the mixture would be too high to allow its spraying out of the can. Furthermore, MDI shows excellent compatibility with the blowing agents (or propellent) generally used.
Free monomeric MDI, however, is highly toxic and the great ease of use associated with OCF formulations is severely outweighed by the care the end-user must show to protect itself from breathing free MDI present in the foaming mixture as it is being dispensed. This is rendered even more critical by the fact that OCF's are often used to fill in cavities which are poorly ventilated and thus exposing the user to great risks.
To solve this problem, DE10311607 proposes to react polyols with a stoechiometric excess of a monomer-free polyisocyanate and then to remove the excess di-isocyanate. Although quite effective, this solution requires an extra step for removing the excess di-isocyanate and an additional thinning agent, dimethyl ether, must be added to reduce the viscosity of the mixture.
Similarly, EP1518874 discloses an OCF composition having a low monomer content, which is obtained from a specific mixture of poly-(phenylene methane poly-isocyanates) (PMDI), and comprising removal of the monomeric isocyanate by distillation.
US2009253819 discloses an OCF composition having a low content of monomeric MDI obtained by reacting a specific mixture of MDI and polymeric MDI having an average functionality of from 2.2 to 5.2. But the preparation of the specific mixture requires the extraction of mixtures of diisocyanates and polyisocyanates having an average functionality of greater than 2 and then freeing them from diisocyanates by distillation, which basically does not remove, but simply shifts the distillation step upstream of the process, compared with the previous disclosures.
WO 2005/007721 discloses an OCF composition having a low content of free MDI, obtained by the use of mixtures of NCO-terminated prepolymers obtained by reaction of polyols and diphenylmethane diisocyanate in stoechiometric excess which had been freed from monomers, and a number of polymeric MDI and diluents. Disadvantageous here are the extremely high viscosities of the starting materials in the absence of diluting monomeric MDI.
It can be seen from this review of the prior art, that it remains a need in the art for a polyurethane composition having a low content of free monomeric MDI, which preparation does not require any monomeric MDI removal step nor the use of additional solvents to compensate for the low amount of monomeric MDI. At the same time, the composition viscosity should be sufficiently low to allow spraying thereof out of a pressurized can, and the NCO content should be sufficiently high to allow a sufficient degree of polymerization to yield the desired properties.
These and other objects of this invention will be evident when viewed in light of the drawings, detailed description, and appended claims.